In rotary air valves such as used in motor vehicle air conditioning, heating and ventilation systems, the periphery of the rotary air valve which may also be called a door is required to seal against the surrounding generally cylindrical case in a consistent manner Moreover, the movement of the valve between positions should be with a minimum of effort. However, the peripheral sealing surface of the valve normally has sliding contact as compared with the twoface contact of a flapper valve. This makes it difficult to control the contact pressure between the seal surfaces while minimizing the effort required to move the valve. Typically, seal materials are selected such as plastic foam, felt, wool pile, and rubber in combination with various retainers in an attempt to control the contact pressure between the seal surfaces. However, the complexity of such combination designs requires added space, is costly and still gives inconsistent sealing and frictional characteristics at various climatic conditions. For example, the different expansion characteristics between the valve, case and seals at temperature extremes causes inconsistent seal performance particularly at low temperatures where it is possible for the valve to freeze in position The functional problem of the rotary valve in turn causes high control lever efforts and potential overload of the valve's operating mechanism.
One solution to the above problems is disclosed in U.S. Pat. 4,683,913 and entails a certain geometrical relationship between the valve seal assembly and the case's sealing surfaces that establishes a predeterminable, consistently controlled contact pressure along the peripheral sealing arrangement. In this arrangement, instead of having the curved sealing surface on the valve case be concentric with the valve axis, it has a fixed radius of curvature whose axis is offset from that of the valve. As a result, there is an eccentric kind of relative motion such that the valve is rotatable to and from its closed position moving toward and away from but not along the seals with the sealing eventually being accomplished during the final stages of closure. And while this solution has proven generally satisfactory, the valve, case and seals require close tolerances to establish the desired sealing force completely along the length of the seal.